There's no grocery store in Niger that specializes in Indian food. However, in Niamey there are several Lebanese grocery stores that may have what you need.
You'll find the shopping in Agadez to be limited and expensive. To my knowledge, there's nothing bigger than a "shop" in Agadez (i.e. no larger grocery stores). You may find some North African food items--particularly from Algeria--but Western food items will be less and almost double the Niamey price.
There are Italian and Lebanese restaurants in Niamey, but I don't think there are any Indian ones. I can't provide you with any info on restaurants in Agadez. I know some of the hotels have some, but there are so few tourists these days who knows what is still operating.